The present invention relates to a respiratory air flowmeter used for medical applications.
Various conventional clinical methods of measuring the flow rate of respiratory air of a patient are known, e.g., (1) a method of using a resistance tube, (2) a method of using a hot-wire anemometer, (3) a method of using a variable orifice, and (4) a method of using a body plethysmograph.
In the method of using a resistance tube, a tube having a resistance arranged midway along its tube path is inserted in a patient, and the flow rate of respiratory air of the patient is measured on the basis of a difference between pressures before and after the resistance. The overall tube is heavy and is difficult to be mounted in the patient. In addition, the tube is expensive. The tube must be cleaned every time it is used and requires a cumbersome handling. Moreover, the tube cannot correctly respond as the speed of respiration is increased. In the method of using a hot-wire anemometer, various disadvantages are posed, e.g., requiring a large, expensive apparatus, calibration for every measurement, difficult handling, special care in measurement of a reciprocating flow, and dangerous for a patient when the hot-wire is disconnected. In contrast to this, the method of using a variable orifice is advantageous in that it has a simple structure. However, it has a critical disadvantage, i.e., low precision. In the method of using a body plethysmograph, a patient enters a box having a volume of about 500 l, and the flow rate of respiratory air is measured on the basis of a change in volume or pressure caused by respiration. This requires a large, expensive apparatus. In addition, since temperatures, atmospheric pressures, and the like must be corrected, and the patient must be moved, measurement cannot be easily performed.